Why is R in the order of Meg ohms in ESD models?

[pawan kumar]

In Human Body Model or Machine model, the Resistance to charge the capacitor is extremely high. Is there any specific reason for it? I assume it is to limit the current inflow. If there is any other specific reason, guys pls let me know.

 

[dick_freebird]

Because a low impedance source will defeat the
time constant of the threat-model network?

 

[pawan kumar]

Hi dick_freebird, thanks for the reply.

It's the discharge time which concerns. Does the charging time constant imply any significance?

 

[dick_freebird]

Charging time impacts only the deliverable repetition-rate.
But depending on the actual topology, the charging circuit
may remain in parallel with the discharge (cap, res, DUT)
loop and affect that time constant, which is spec-defined
and must toe the line. A Mohm charge resistor against the
1.5Kohm HBM series resistor contributes < 1% time constant
error. A 1K charge resistor would cut tau by more than half
if the HV source - resistor path does not have a switch to
open it before connecting the output to the load.